Polyolefin resin compositions are excellent in a variety of performances such as mechanical strength, heat seal performance, chemical resistance and food hygienic properties, and therefore they are widely used as materials of polymer films, and so on. In recent years, these polyolefin resin compositions have begun to be used for packaging of more expensive contents and for members of expensive industrial products, so that demand for their quality has become severer than before.
If contaminants such as gel-like carbonized polymers or dust are contained in a polyolefin resin composition, a circular defect, which is called a “fisheye” because of its similarity to the shape of a fisheye, is caused on the surface of a molded film of the polyolefin resin composition, which results in deterioration in appearance.
On account of this, as a method for removing contaminants from a polyolefin resin composition, methods in which the composition is filtered through a metal mesh, sintered metal filters such as a metal fiber sintered body and a metallic powder sintered body are known (see, for example, Patent Documents 1, 2).
A filter excellent in filtration accuracy is usually used in the filtration method mentioned above so as to sufficiently reduce the occurrence of fisheyes. When such a filter is used, however, excessive load is imposed on an extruder and therefore productivity deteriorates. Accordingly, a filtration method using a leaf disk type filter in which the filtration area is large enough but does not cause the filter facilities to become excessively large is proposed (see, for example, Patent Document 1).
In addition, demand for various physical properties and workability of a polyolefin resin composition to be achieved at high levels has increased with the diversification of the product thereof. Polyolefin resin compositions containing a plurality of polyolefin ingredients are being examined in order to meet such demand. Since fisheye due to poor dispersion of respective ingredients is liable to occur in such a polyolefin resin composition, technique to use various kinds of filters is proposed (see, for example, Patent Document 3).    Patent Document 1: Japanese Patent Laid-Open No. 2006-88081    Patent Document 2: Japanese Patent Laid-Open No. 09-38423    Patent Document 3: National Publication of International Patent Application No. 2000-511967